Module for furnace walls totally wrapped with thermally combustible material

ABSTRACT

The invention relates to the insulating of furnace walls. In accordance with the invention insulating modules are secured to the wall with cement. According to a preferred feature of the invention the modules are contained under elastic compression in a combustible netting envelope.

This is a division, of application Ser. No. 707,108, filed July 20,1976, now abandoned.

This invention relates to the modular construction of furnaces, that isto say, the construction of the internal wall of a furnace from discreterelatively small modules of refractory material. The invention isconcerned both with the structural and insulating modules which areattached to the cold face of a furnace, i.e. to the basic metalconstruction of the furnace itself and to purely insulating andrefractory modules, hereinafter referred to as "biscuits", which aresecured to an existing hot face made up, for example, of refractorybricks. In the latter case, the invention envisages reducing the thermalinertia, i.e. the amount of heat required to raise the walls of thefurnace to operating temperature by the application of light highlyrefractory "biscuits".

In the modular construction of furnace walls it has been proposed tosecure each module to the basic metal furnace wall construction, the"cold face", or to the hot face of refractory bricks by some physicalmeans such as, for example, brackets or rivets.

According to the present invention the surface of the module which is tobe fixed to the furnace hot or cold face is tacky i.e. when applied toan inclined or overhanging surface, it provides sufficient adhesion notto fall under gravity. Further, according to the invention, afterapplication the tacky surface is caused or allowed to set into a stableheat resistant and refractory condition which firmly and permanentlyadheres to the furnace wall. The present invention envisages severalmeans for achieving this setting:

Firstly, the invention provides what may be broadly described as achemical procedure, wherein the tacky rear surface layer is contactedwith a reagent, either before or after application to the furnace wall,which causes it to set. This can be achieved by mixing a self-settingcement, applying it to the back of the module and placing the module onthe wall and allowing the cement to set. The setting can be acceleratedby preheating the furnace face. Alternatively the module can be coatedwith a cement which immediately prior to placing the module is treatedwith a setting agent.

An alternative procedure envisaged is to apply the tacky module to thehot or cold face and subsequently to cause setting by introducing thesetting agent. This can be achieved by contacting the module as a wholewith a gas vapour or mist of liquid droplets and allowing the gas vapouror mist to impregnate the module and react with the rear surface. Thesetting agent can, as an alternative, be introduced by injection throughthe module. Refractory binder techniques wherein a binder is hardened byan accelerator can thus be employed, the binder being used to impregnatethe module or module part and the accelerator applied by injection or asa gas, vapour or mist. Thus in an embodiment an alkaline alkali metalsilicate system with a suitable refractory filler providing an adhesivecement, is hardened by contracting the cement with an acid or anacid-producing gas such as carbon dioxide, sulphur dioxide orhydrogen-chloride. In a further possible system a cement comprising asthe active ingredient an aluminium hydroxyhalide is hardened bycontacting it with ammonia or an amino-alcohol. Thus the inventionprovides alkali cements hardened by acids, and acidic cements hardenableby alkalis. In these preferred techniques the module can be applied by asuitable applicator which embraces the module closely and is connectableto a source of gas, vapour or mist. In the case of injection a toolhaving generally similar dimensions to the module has a plurality ofinjection nozzles on the front face and a connection to a source ofliquid. Such a tool enables liquid to be injected through the module tocontact substantially the whole rear tacky face.

The invention also envisages hardening a tacky cement by some physicaltechnique such as electrical, microwave or ultrasonic energy applied byappropriate electrodes. In an embodiment an aluminium heating element isprovided under the tacky layer. An energising device has conductiveprobes which can be inserted through the module to make electricalcontact with the heating element. The heating element can thus beenergised from the front of the module to set the cement. Thesetechniques are a development of the simple warming techniques referredto previously. A further possibility is to set a tacky cement byinjection from the front of the module of a desiccating agent such assilica gel.

Although the invention is applicable to modules of any appropriaterefractory composition it finds particular application to modules whichconsist of or comprise fibres orientated so that the fibres are at rightangles to the plane of the module and end-on to the furnace wall.

The choice of fibre will be determined by the specified operatingtemperatures of the furnace. Thus for temperatures of up to 900° C.mineral wool fibres may be used, for temperatures between 900° C. and1200° C. aluminosilicate ceramic fibres will normally be used whilst fortemperatures between 1200° C. and 1600° C. alumina fibres should beused.

In the case of a module made up of "end-on" fibres the fibres do nothave sufficient coherence to accept readily an adhesive coating.

The present invention thus proposes wrapping a biscuit, preferably madeup of end-on fibres, with combustible netting thereby rendering thebiscuit coatable with an adhesive. The netting is held in position withglue applied to the faces of the biscuits, the glue usually beingorganic and having sufficient strength to provide, with the netting, atough flexible outer skin. The mesh of the netting should besufficiently small to ensure the physical integrity of the module andsufficiently large to allow effective contacting of the fibres withcement. A 1/8"-1/4" mesh has been found effective. Upon first firing ofthe furnace the netting is burnt away. If the fibres are held in elasticcompression recovery on burning off the mesh causes each module to pressagainst adjacent module(s) thereby enhancing the coherence of the wallas a whole. A further advantage of the netting wrap is that, whenapplying adhesive cement with a bladed tool, such as a trowel, the toolblade can readily be wiped clean of adhesive on the netting.

In embodiments of the invention wherein the combustible netting is notbeing used some other combustible means will preferably be provided forholding the module in elastic compression.

The invention contemplates insulating modules formed of one or aplurality of insulating layers. Also a module with a flexible or a rigidbacking.

The present invention further provides a circular biscuit formed bywinding a strip or strips of "end-on" fibres into a spiral. The circularbiscuit thus formed is bound into position with a combustible netting,the netting being held in position with radial strips of glue. Largecircular pieces fabricated in this way may be cut into sections andre-assembled on installation.

Various embodiments of the invention will now be described in theaccompanying drawings wherein:

FIG. 1 is a diagrammatic view in section and a fragmentary planillustrating a biscuit in accordance with the invention;

FIG. 2 in section and in plan illustrates the netting wrapping of abiscuit; and

FIG. 3 in plan illustrates a circular biscuit.

Referring now to FIG. 1 a biscuit 3 typically 225 mm square and 50 mmthick made up of strips 3a of "end-on" fibres is secured to a furnacewall 1 by a layer 2 of cement. The end-on configuration of individualfibres is diagrammatically illustrated by the shading. The plan viewillustrates how the end-on fibre strips 3a are preferably orientated inan array of biscuits so that no continuous composite strips are defined.

FIG. 2 shows a biscuit wrapped in netting 4 by a strip-shaped bondinglayer 5 of organic glue i.e. combustible to dissipate as gaseousproducts.

FIG. 3 shows a circular biscuit 6 formed by winding a strip 7 of end-onpieces into a circular structure somewhat similar in shape to a large`Catherine Wheel` firework. The wound strip is wrapped in netting 8secured by radially strip shaped bonding layers 9 of glue.

German laid open specification No. 2,513,740 (Zirconal ProcessesLimited) and British Patent Specification No. 1,427,708 (ZirconalProcesses Limited) disclose useful binder/accelerator systems whilstBritish Patent Specification No. 1,423,167 discloses a cement forsticking ceramic fibres.

FIG. 4 of the accompanying drawings shows the application of theinvention to the lining of a flue made up of a plurality of metalcylindrical sections 10 joined end to end. To facilitate joining eachend of each section 10 has an annular radially outwardly extendingflange 11 (FIGS. 4(a) and 4(b)). A sheet 12 of expanded metal mesh isformed into a cylinder to fit the internal diameter of the section 10,the butting edges of the sheet being tack welded together to form thecylinder. Strips 13 of ceramic fibre are laid transversely on a broadstrip of netting 14, the strips 13 being secured to the netting 14 withorganic glue (FIG. 4(c)). Each strip 13 is formed of fibres in the"end-on" orientation as previously described so that in use the fibresextend radially inwardly in use. The length of the netting, i.e. thedistance X equals the internal circumference of the expanded metalsleeve 12 whilst the width of the netting, i.e. the length of the strips13 is slightly longer than the length of the section. The netting 14carrying the strips is formed into a cylinder this having the effect ofcompressing the inner parts of the ceramic fibre strips 13 and thesleeve thus formed FIG. 4(d) is inserted into the metal sleeve. Theexternal surface of the composite metal mesh and fibre sleeve thusformed is then coated with refractory cement 25 and inserted into theflue section to produce the assembly shown in FIG. 4e. When adjacentassemblies are secured together end on by the flanges 11 projectingfibre strips 13 are compressed together.

FIG. 5 of the accompanying drawings shows the invention as applied tobuilding up a furnace wall on a steel casing 15. Here in addition to themodule 16 applied with a tacky cement layer 17 in accordance with theinvention an intermediate layer of insulating material is applied. Thusspecifically the steel casing 15 has pinned thereto using suitable pins21 anchors 18 and washers 19 insulating block material 20. In thedescribed and illustrated embodiment the insulating blocks 20 arediscrete items having sufficient integrity to be fixed in the mannerdescribed. When the supplementary insulating material lacks suchphysical integrity and is for example mineral wool it can be wrapped forexample in expanded metal sheet and either anchored or cemented inposition.

Examples of compositions hardenable by an acid producing gas andsuitable for use according to the present invention are as follows:

    ______________________________________                                        Composition No. 1                                                             ______________________________________                                        Potassium silicate solution `Pyramid`                                                                  100 grams                                            No. 53 This solution has the following                                        properties:                                                                   Wgt.ratio SiO.sub.2 : K.sub.2 O 2.48:1                                        Mol.ratio SiO.sub.2 : K.sub.2 O 3.89:1                                        SiO.sub.2 = 21.30% w/w K.sub.2 O = 8.60% w/w                                  Aminoalcohol              10 grams                                            `Molochite` (Sintered China Clay)                                             120 grade                100 grams                                            Mica                      10 grams                                            Tabular alumina - 24 & 48 grade                                                                         15 grams                                            ______________________________________                                    

The addition of the tabular alumina is not essential. The aminoalcoholis monoethanolamine or tris (hydroxymethyl) methylamine. It may, ifdesired, be replaced by 10 ml of aqueous ammonia solution sp.gr. 0.88.

    ______________________________________                                        Composition No. 2                                                             ______________________________________                                        `Claysil` No.1 (Claysil is a dispersion                                                                100 grams                                            of clay in sodium silicate solution having                                    the following properties:                                                     Wgt. ratio SiO.sub.2 : Na.sub.2 O 3.30:1                                      Mol. ratio SiO.sub.2 : Na.sub.2 O 3.41:1                                      Na.sub. 2 O = 6.57% w/w                                                       SiO.sub.2 = 21.70% w/w                                                        Total solids = 51.1% w/w                                                      Viscosity at 20° C. 250-1200 centipoises                               Ammonia solution 0.88 sp.gr.                                                                           10 ml                                                ______________________________________                                    

The invention will now be described with reference to the attachment ofceramic fibre thermal insulation to a furnace wall. It is essential touse only a thin layer of a composition such as composition No. 1 or No.2. The furnace wall is coated with a thin layer, thickness about 0.1inch, with Composition No. 1 or Composition No. 2. Strips of ceramicfibre, oriented so that a substantial proportion of the fibres areend-on to the supporting surface, are embedded in the composition, whichis now hardened by the action of carbon dioxide gas.

"Biscuits" suitable for use in the furnace wall construction may besupplied with the hot face protected by corrugated cardboard and thesides protected by paper. The reverse or cold face may also be coveredwholly or in part by a backing material, to retain the shape duringtransit or insulation, or to form a key with the cement. A suitablebacking material is glass fibre, covering about 20% of the area of thecold face. Either cold face or furnace wall may be coated with thecompositions, or both may be coated.

FIG. 6 of the accompanying drawings shows a tool suitable for use inattaching a "biscuit" to a furnace wall. The tool comprises a box 22made of light gauge metal, to accept a "biscuit" as an easy fit, with ahollow tube 23 to function as a handle and as an inlet for carbondioxide gas. A light gauge metal plate 24 can be provided to locate thebiscuit. The biscuit is placed in the box, with the cold face uppermost,then a thin layer of Composition No. 1 or No. 2 is applied. After cementcoating, the biscuit is located in the required tubes, which passthrough the fibres and make contact with the cement composition. Theinlet tube for carbon dioxide gas is above this plate.

In the following examples, the tacky rear surface layer mentionedpreviously is one of the compositions described in U.K. Pat. No.1,423,167. Of the compositions described in U.K. Pat. No. 1,423,167those using Laponite SP are preferred. In the following, thesecompositions will be referred to as "sol cements", for convenience.

EXAMPLE A

Sol cement was applied to a furnace insulating brick (hot face) and to agroup of ceramic fibre strips constituting a module. Zirconium acetatesolution was poured over both cement surfaces, then the surfaces wereimmediately joined. Three minutes after the surfaces were joined thecement had set. The adhesion between brick and ceramic fibres was good.

No advantage was gained by first treating the fibre strips withzirconium acetate solution, then applying the sol cement to the surfaceof the fibres, followed by pouring zirconium acetate solution over thecement. Priming the brick surface with zirconium acetate solution gaveno advantage.

EXAMPLE B

The tacky rear surface layer was formed by immersing the ceramic fibrestrips in zirconium acetate solution. A layer of sol cement was appliedto a furnace insulating brick. Then the ceramic fibre strips(impregnated with zirconium acetate solution) were applied to the layerof sol cement. The cement set in about 3 minutes after the surfaces werejoined, but the adhesion was not so good as in (A). The zirconiumacetate solution used in (A) and (B) contained 20% ZrO₂ w/w.

A suitable zirconium acetate solution is described in British AluminiumCompany's data sheet No. 431.

EXAMPLE C

An alternative to zirconium acetate solution is an acid hydrolysed ethylsilicate solution. The preparation of a suitable acid hydrolysed ethylsilicate solution is described in British Patent Specification No.1,356,249.

Sol cement was applied to a furnace insulating brick and to a group ofceramic fibre strips. Acid hydrolysed ethyl silicate solution was pouredover both cement surfaces, then the surfaces were immediately joined.The cement had set three minutes after the surfaces were joined. Theadhesion between brick and ceramic fibres was good and improved onstanding.

An alternative procedure is to apply the sol cement to the surface ofthe ceramic fibre, then place the coated surface on the wall of anexisting furnace. The cement is hardened by applying zirconium acetatesolution or acid-hydrolysed ethyl silicate solution to the cementsurface. A "grease gun" is suitable for this.

The sol cement may be hardened by physical means, for instance byapplication of local heat. One way of applying local heat is to embed aheating element in the cement. This heating element can conveniently bemade from an electric resistance wire suitable for operating at hightemperature. "Nichrome" wire is suitable.

We claim:
 1. A module for application to a furnace or oven wall toreduce the thermal inertia thereof, such module being made up ofrefractory fibers disposed with their axes aligned at right angles tothe plane of the module so that they are end-on to the furnace or ovenwall, the fibers of said module being totally wrapped and held inelastic compression by a wrapping of thermally combustible materialwhich on first firing of the furnace or oven after application of themodules, is thermally destroyed so that the module is released from thecompression.
 2. A module according to claim 1 wherein the said materialis a netting wrap.
 3. A module according to claim 1 wherein the fibersare ceramic fibers.